On Joule heating of the equatorial electrojet E-region

Simultaneous data on electron density, electron temperature and current density obtained from a rocket borne Langmuir probe, a glass-sealed Langmuir probe and a proton precession magnetometer flown from Thumba (geomag. lat. 0.99°S, geomag. long. 146.79°E, magnetic dip 0°47'S) have been used to calculate the Joule heating in order to assess whether it contributes significantly to the thermal imbalance in the E-region. It is envisaged that the changes in electron temperature are partially brought about by changes in collision frequency and the energy loss factor. It is found that the Joule heating alone is not sufficient to explain the observed differences in electron and neutral gas temperatures. The inclusion of photoelectron heating and adjustments of profiles of the collision frequency and the energy loss factor bring the computed temperature differences closer to the observed differences.     

HF scattering induced by field-aligned irregularities in the equatorial E-region during total solar eclipses

In this paper, results and analyses of solar eclipse effects on the lower ionosphere are presented. After the first contact of the total eclipse on 16 February 1980, an absorption increment of 12 dB was observed. At the same time, the frequency of amplitude fading increased largely and Doppler frequency shift disturbances appeared. The calculation of signal strength is carried out by means of Booker's scattering theory, supposing an outer scale T_o = 1000 m and an inner scale T_i = 5 m, of space scale spectrum of field-aligned irregularities in the equatorial E-region. The calculated results agree fairly well with observations. Results showed that, because of the formation of lower ionospheric field-aligned irregularities in the course of the obscuration of solar local ionization source, radio wave scattering was strengthened.     

Nonlinear solitary wave in the ionospheric E-region

A nonlinear wave equation for a solitary one-dimensional inhomogeneity is studied. It is similar to the diffusion equation with the diffusion coefficient depending on the phase velocity. The phase velocity depends, in turn, on the electron density. A weak inhomogeneity moves with the velocity close to that of the electric drift. If this velocity exceeds the ion acoustic speed the effective diffusion is negative, and the inhomogeneity grows and contracts. The velocity of the growing inhomogeneity becomes smaller. It absorbs weaker and faster moving inhomogeneities from the back side. In a stationary regime, the ionosphere will be filled with rare but strong inhomogeneities with sharp back sides.     

The effects of the resonance broadening of Farley-Buneman waves on electron dynamics and heating in the auroral E-region

Several theories have now been developed which deal with the saturation mechanism of the Farley-Buneman instability and also the heating effect of Farley-Buneman waves on E-region electrons. Various related phenomena, such as anomalous collisions, anomalous cross-field diffusion and non-zero aspect-angle propagation have been treated. In this paper, it is demonstrated how several of these features may be included in a single resonance broadening formalism. Using this approach, the effects of Farley-Buneman waves on the heating and dynamics of background ionospheric electrons is discussed. In addition the effects of anomalous parallel diffusion in the context of E-region Farley-Buneman waves is treated for the first time.     

Empirical electron recombination coefficients in the D- and E-region

Effective electron recombination coefficients were deduced from measured ion production rates and plasma densities of 29 D-region rocket flights. They were grouped according to day and night conditions and arranged vs. ion production rate. Corresponding results from an ion-chemical model are presented for comparison.     

Electron temperature enhancements in the polar E-region measured with EISCAT

We have found electron temperature enhancements up to 1000K at 110 km altitude using the EISCAT multipulse method which allows high spatial resolution measurements within E-region. This electron temperature enhancement which is closely related to the d.c. electric field strength, is in good agreement with theoretical estimates based on wave heating. The results of the measurements are presented together with a discussion of the electron gas heating, its height variations and its difference in the eastward and westward electrojet.     

Winds and electric fields in the upper E-region over Malvern

Some preliminary hourly values of total ionic velocity in the upper E-region (122–144 km) are presented and the magnetic meridian component of horizontal neutral wind, averaged from observations on 14 days spread over about two years, is studied. The semi-diurnal variation is found to be predominant at 122 km, almost exactly in phase with the same component at St. Santin (France). Knowledge of the complete ionic velocity allows the rigorous calculation of the meridian perpendicular component of the electric field, some preliminary values of which are presented.     

Direction of arrival of radio waves reflected from the E-region

A simple system for measuring the direction of arrival of continuous high frequency radio waves reflected obliquely from the ionosphere is described. Results obtained over a near vertical path are used to evaluate changes in the electron density gradients (tilts) in the E-region. Wave interference on the transmission due to multiple hop or ground wave propagation is discussed.     

Adiabatic and isothermal ion-acoustic speeds of stabilized Farley-Buneman waves in the auroral E-region

The influence of ion and electron energetics on the propagation speeds of stable Parley Buneman waves which are excited by E × B drifts in the auroral E-region is studied theoretically in the fluid limit, with the effects of anomalous collisions on electron thermal conduction included for the first time. In particular, the ratio of the phase speed of waves, stabilized by enhanced diffusion effects, to the isothermal ion-acoustic speed are calculated for realistically modelled E-region ion and electron temperatures, as functions of altitude, flow velocity and wavelength. It is found that the phase speeds of these stabilized waves begin to increase above isothermal ion-acoustic speeds as wave frequencies increase to values where they are comparable with the electron inelastic collision frequency. However, at still higher frequencies their phase speeds tend to fall back towards their isothermal values due to the increasing effects, with increasing wavenumber, of electron thermal conductivity. It is also found that the phase speeds are not always isotropic with respect to flow angle. The relationship between the predictions of the present fluid theory and a previous kinetic theory calculation is also briefly discussed.     

Tests of an ion-chemical model of the D- and lower E-region

A relatively simple steady-state ion-chemical model for the non-auroral D- and lower E-regions is presented. Solar fluxes and the neutral atmospheric constituents are taken from up-to-date literature. The model results are systematically compared to measurements, namely: ion composition, electron density vs solar zenith angle, solar activity, season and latitude. Although the agreement is not always good, it is believed to exceed that of purely statistical models.     

Long-period motions in the equatorial mesosphere

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3–10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2°N, 157°W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9°N) and Ascension Island (8°S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January–June) than in the second half-year (July–December). An annual cycle dominates the meridional winds with maximum velocities (5–10m s⁻¹) attained at about 90km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3–10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.     

Coherent radar (SABRE) studies of dynamical processes in the auroral E-region

In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.     

Topside irregularities in the equatorial ionosphere

This paper is a brief review of ionospheric irregularities in the equatorial topside ionosphere. Results from topside sounders, direct measurement satellites, and the Jicamarca incoherent scatter radar are discussed. Scintillation observations and theories of irregularities are not discussed in detail as these are the subject of other review papers. Many of the phenomena detected in the topside ionosphere are related to bottomside irregularities, commonly known as spread-F. These include aspect-sensitive scattering observed on topside sounders, significant concentrations of Fe⁺, electrostatic turbulence and the topside irregularities detected by the Jicamarca radar. Satellite measurements show that the irregularities in electron concentration have amplitudes which increase almost linearly with wave-length over the range 70m to 3km. Duct irregularities detected by the topside sounders and some wavelike irregularity structures detected occasionally by direct measurement satellites may be separate from the general spread-F phenomenon although this has not definitely been established.     

Generation of stationary irregularities due to heating of the high-latitude ionospheric E-region

The nonlinear equations describing the generation of artificial irregularities in the E-region of the high-latitude ionosphere due to Joule electron heating under the action of high-power radio waves are derived, including the low-frequency nonlinearities, and investigated in the three-wave approximation. Expressions for the stationary spectra of the short wavelength two-stream and long wavelength gradient-drift and current-convective irregularities are presented. The typical saturation amplitudes of the density fluctuations are obtained.     

Inter-relations between current and electron density profiles in the equatorial electrojet and effects of neutral density changes

A simple model of the equatorial electrojet is used to try to reproduce observed current density profiles and it is found that an increase in neutral density is required. The effects of neutral density changes of various kinds are investigated. Changes in the electron density profile due to the j × B force are found to be fairly small and, in the cases studied here, are decreases at all heights.     

Electrodynamic response of the middle atmosphere to auroral pulsations

On the nights of 21 and 28 October 1987, two Nike Orion payloads (NASA 31.066 and 31.067) were launched from Andøya, Norway, as part of the MAC/EPSILON campaign, to study the effect of auroral energetics on the middle atmosphere. Each payload carried detectors to measure relativistic electrons from 0.1 to 1.0MeV in 12 differential energy channels, and bremsstrahlung X-rays from >5 to >80keV in 5 integral channels. In addition, instrumentation to measure bulk ion properties and electric fields was also carried by these and/or near simultaneous flights. Flight 31.066 was launched during the recovery phase of a moderate magnetic substorm, during relatively stable auroral conditions. Flight 31.067 was launched during highly active post-break-up conditions during which Pc 5 pulsations (> 150s period) were in progress. The energetic radiation of the first event was composed almost entirely of relativistic electrons below 200 keV with negligible contributions from bremsstrahlung X-rays, while the radiation of the second event was dominated by much softer electrons ( < 100 kcV), which produced high X-ray fluxes that exceeded the cosmic ray background as an ionizing source down to altitudes below 30 km. Simultaneous conductivity measurements during both events show consistency with the ionizing radiations, with the pulsation event producing free electrons down to 55 km. far below their expected altitude range during night-time. These comparisons are discussed to evaluate the impact of such events on the middle atmosphere.     

Meridional electric currents in the equatorial F-region

A study of the boundary conditions for the equatorial thermospheric transport equations by the authors has led to the theoretical prediction of the vertical electric field at the base of the F-region. Earlier, this result was applied to the calculation of the zonal wind field in the equatorial F-region. In this work, the aforementioned model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat.     

Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.     

Lunar modulations of the equatorial electrojet

An attempt has been made to reproduce the counter electrojet (CE) in the equatorial dynamo by considering neutral winds with solar (1,–2), (2, 4), (2, 2) and lunar (2, 2) tidal modes as well as a constant electrostatic field (E_y). The daily variation of conductivity (σ) is assumed to consist of steady (average), diurnal and semi-diurnal components. An equation governing the relationship between j_y (jetcurrent), E_y, σ and wind is given, and this equation is then used to describe diurnal, semi- and ter-diurnal variations of j_y separately. It is found that: (1) the lunar tide is relatively powerful in affecting semi- and ter-diurnal components of j_y; (2) such a possibility is a maximum for the afternoon CE near new and full moon and (3) the morning CE is likely to occur at lunar age between the new and full moons. From this theory, the seasonal characteristics and the solar activity dependence of CE are demonstrated to be predictable.     

Solar flare effects on zonal and meridional currents at the equatorial electrojet station,Annamalainagar

During the normal electrojet period, a solar flare produces a positive change in the horizontal (H) field, negative changes in the eastward (Y) field and a negative change in the vertical (Z) field at a northern electrojet station. On average, the ΔY is about 40% of ΔH. During a counter electrojet period, ΔH, due to a solar flare, is negative and ΔY and ΔZ are positive. During a partial counter electrojet period, ΔH may be smaller at equatorial stations compared with other low latitude stations, and ΔY may be positive, or sometimes of very small magnitude. The observed change of ΔY at an electrojet station is suggested to be the combined effect of the flare on the associated Sq current system and on electrojet related meridional currents. These data confirm the seat of the equatorial meridional current to be in the ionospheric E layer.